1. Field of the Invention
The present invention relates to a process for producing 2,6-dimethyl-1,3,6-octatriene by linear dimerization of isoprene. More particularly, it is concerned with the use of novel catalyst compositions comprising specific zirconium compounds, aluminum compounds, and coordinating compounds.
2. Description of the Prior Art
For producing 2,6-dimethyl-1,3,6-octatriene from isoprene, there have been proposed a method using Ziegler type catalysts wherein titanium compounds are used, as described in French Pat. No. 1,393,071, Quarterly of the Government Industrial Research Institutes, Osaka, 15, 271, Makromol. Chem., 123, 274 (1969), etc., a method using vanadium based catalysts as described in Japanese Patent Laid Open No. 5704/1973, etc., and so on. In accordance with these methods, however, cyclic dimer, trimer and the like are by-produced and good results cannot be obtained.
Furthermore, as a method for selectively producing 2,6-dimethyl-1,3,6-octatriene, the use of zirconium based catalyst is described in Bull. Chem. Soc. Japan, 42, 2303 (1969), Japanese Patent Laid Open Nos. 34303/1972 and 5706/1973, etc., and the use of hafnium-based catalyst is described in Bull. Chem. Soc. Japan, 42, 1383 (1969), etc.
These catalysts, however, could incur certain problems, such as that the production thereof is, as described in J. Chem. Soc., 1952, 2032 to 2035, etc., complicated in that ammonia is used as a dehydrochlorination agent, precipitated NH.sub.4 Cl is filtered and washed as a post-treatment, and so on, and that conventional Zr(OR).sub.4 is readily hydrolyzed with the moisture contained in air and is polymerized into resinous compounds.
Furthermore, the use of these catalysts is industrially disadvantageous in that the catalyst cost is high because sufficient activity cannot be obtained unless a large amount of the catalyst is added to the monomer feed.
It has been previously found through our investigation of the oligomerization of isoprene by the use of a zirconium based catalyst that a zirconium alkoxychloride-based catalyst overcomes the above mentioned drawbacks (see U.S. Pat. No. 3,689,585, British Pat. No. 1,298,609, and Japanese Pat. Publication No. 104,020/1969).
As a result of further investigation of the improved catalyst, we have now discovered a novel catalyst which can be produced at much lower cost and much easier than conventional zirconium based catalysts.